KEP-5339: add additional cluster-specific auth info field to the cluster profile object

keps/sig-multicluster/5339-clusterprofile-plugin-credentials/README.md

@@ -224,7 +224,8 @@ controller, applications or consumers without requiring changes. It also cannot
 is not sensitive.
 
 The definition is as follows:
-```
+
+```golang
 type CredentialProviders struct {
   // +listType=map
   // +listMapKey=name
@@ -237,7 +238,7 @@ type CredentialsType string
 // CredentialsConfig gives more details on data that is necessary to reach out the cluster for this kind of Credentials
 type CredentialsConfig struct {
   Name string // name of the provider type
-  Cluster *Cluster // Configuration to reach the cluster (endpoints, proxy, etc) // See following section for details.
+  Cluster *Cluster // Configuration to reach the cluster (endpoints, proxy, etc) // See the sections below for details.
 }
 ```
 
@@ -290,10 +291,85 @@ In this structure, not all fields would apply, such as:
 
 * `CertificateAuthority`, which points to a file (and a ClusterProfile doesn't have a filesystem)
 
+##### About the `Extensions` field
+
+The `Cluster` type defined in `client-go` ([link](https://github.com/kubernetes/client-go/blob/d32752779319f587c42ff9edbc6ed533575f2136/tools/clientcmd/api/types.go#L69)),
+which is also a struct in use by the `CredentialsConfig` type from this KEP,
+features a field, `extensions`, for holding additional information about the cluster, with each extension associated with a name.
+[KEP-541](https://github.com/kubernetes/enhancements/blob/master/keps/sig-auth/541-external-credential-providers/README.md) further reserves a name,
+`client.authentication.k8s.io/exec`, for per-cluster additional information for authentication exec plugins.
+Per KEP-541's explanation, if the `client.authentication.k8s.io/exec` extension has been set in the `Cluster` struct, the data shall be
+parsed and populated into the `Config` field of the `ExecConfig` struct (also defined in the `client-go` package, see [link](https://github.com/kubernetes/client-go/blob/d32752779319f587c42ff9edbc6ed533575f2136/tools/clientcmd/api/types.go#L209)).
+If the `ExecConfig` struct has its `ProvideClusterInfo` flag set to true, the `client-go` package, upon invocation of an exec plugin, will build a
+`Cluster` struct (from the `client.authentication.k8s.io` API group, see [link](https://pkg.go.dev/k8s.io/client-go/pkg/apis/clientauthentication#Cluster)),
+which includes the parsed extension data (`Config` field in the `ExecConfig` struct), and save it to an environment variable, `KUBERNETES_EXEC_INFO`.
+
+For the workflow defined in this KEP, however, as described earlier in this document, it is up to the community-provided library to build a 
+`rest.Config` and return it to the caller based on some user-supplied exec plugin information, which already features an `ExecConfig` struct
+(with the path to the exec plugin, arguments, environment variables, etc.) before it reads a `ClusterProfile` object; if the `ClusterProfile` object
+has extensions set, it might be in conflict with the `ExecConfig.Config` field that the community-provided library sees (typically, as KEP-541 dictates, data
+in `ExecConfig.Config` should be sourced from `Cluster.Extensions`). To address this conflict, this KEP proposes that:
+
+* If the `Cluster` struct in a `ClusterProfile` object already features some extension data under the name `client.authentication.k8s.io/exec`, the community
+library will overwrite the `ExecConfig.Config` field with the data from the `ClusterProfile` object. It is up to the `ProvideClusterInfo` flag, which is
+set exclusively on the community-provided library side, to decide whether this extension data will be provided as a part of the environment variable, `KUBERNETES_EXEC_INFO`.
+
+In addition, as the extension data under the name `client.authentication.k8s.io/exec` is provided in the form of a `runtime.RawExtension` struct, and the
+`ExecConfig.Config` field accepts only a `runtime.Object` interface, the community-provided library will attempt to bridge the two by saving the extension data as a
+`runtime.Unknown` struct. It is up to the user to ensure that the extension data is of a correct format that can be serialized/deserialized when being saved as the
+`KUBERNETES_EXEC_INFO` environment variable, and can be processed properly by the target exec plugin. The community-provided library will not perform
+additional validation on the extension data.
+
+##### Supplying additional CLI arguments and environment variables to the exec plugin
+
+The current `Extensions` interface for passing additional cluster-specific authentication information might not be very easy for users to use. The presence
+of the `KUBERNETES_EXEC_INFO` environment variable is purely optional; if the `ProvideClusterInfo` flag is unset (the default), this variable will be absent.
+There are a few cases where the variable cannot be easily set: for example, the `KUBERNETES_EXEC_INFO` environment variable does feature the CA bundle for the
+target cluster, which can potentially get quite large; depending on the target environment, it might not be OK to write such data as an environment variable.
+Exec plugins are not mandated to support this environment variable either; even for those that do read this environment variable, it is not guaranteed that any
+embedded extension data will be properly extracted.
+
+On the other hand, it is quite common for multi-cluster users to have a need for specifying cluster-specific information when performing the authentication
+workflow: the authentication solution in use by a cluster might be expecting a token of a specific audience, or a token from a specific identity. Most, if not
+all, exec plugins would expect such information from the CLI arguments or some environment variables. In a single-cluster setup, it is trivial to set them up, as the
+user has direct control over the `ExecConfig` struct; however, for the multi-cluster setup, since the `Cluster` struct does not feature any fields for additional
+CLI arguments or environment variables, it would be fairly difficult to achieve this when using the Cluster Profile API and community-provided library.
+
+To address this decifiency, we further proposes that:
+
+* this KEP reserve a name in the extensions, `multicluster.x-k8s.io/clusterprofiles/auth/exec/additional-args`, which holds additional CLI arguments that need to be
+supplied to the exec plugin when the Cluster Profile API and community-provided library are used for authentication.
+
+    If an extension under this name is present, the community-provided library will extract the data, and append the additional arguments to the `ExecConfig` struct
+    (specifically the `ExecConfig.Args` field) that will be used to prepare the `rest.Config` output. The arguments will then be used to invoke the exec plugin.
+
+    The additional arguments should be represented as a string array in the YAML format.
+
+    For simplicity reasons, the community-provided library will not perform any de-duplication on the CLI arguments after appending the additional arguments.
+
+* this KEP reserve another name in the extensions, `multicluster.x-k8s.io/clusterprofiles/auth/exec/additional-envs`, which holds additional environment variables
+that need to be supplied upon calling the exec plugin when the Cluster Profile API and community-provided library are used for authentication.
+
+    If an extension under this name is present, the community-provided library will extract the data, and add the additional variables to the `ExecConfig` struct
+    (specifically the `ExecConfig.Env` field) that will be used to prepare the `rest.Config` output. The variables will then be set when invoking the exec plugin.
+
+    The additional environment variables should be represented as a mapping between strings in the YAML format.
+
+    The community-provided library will de-duplicate the list of environment variables when adding the additional variables; if two entries are present under the
+    same name, the one from the extension will prevail.
+
+###### Security concerns
+
+With the addition of newly reserved extensions, understandably there might be situations where users might want to block additional CLI arguments or
+environment variables from being set due to security reasons. To solve this, the KEP proposes that the community-provided library implementation must provide
+two flags, `allowAdditionalCLIArgsExtension` and `allowAdditionalEnvVarExtension`, that control whether additional CLI arguments or environment
+variables will be read from a `ClusterProfile` object respectively. A reserved extension will be not processed if the corresponding flag is unset. By default both flags should be unset.
+
 
 #### ClusterProfile Example
 
 Example of a GKE ClusterProfile, which would map to a plugin providing credentials of type `google`:
+
 ```
 apiVersion: multicluster.x-k8s.io/v1alpha1
 kind: ClusterProfile